1. Field of the Invention:
The present invention relates generally to the field of tube-type, gravity-flow mixing apparatus for particulate matter, particularly, retorted oil shale.
2. Discussion of the Prior Art:
Organic-rich shales (oil shales), which are found on every continent in sediments ranging in age from Cambrian to Tertiary, represent a vast source of fossil fuel. Oil shales capable of producing from 25 to 100 barrels of oil per ton of shale have recently been estimated to represent an oil reserve of about 9.times.10.sup.11 barrels. If all oil shales capable of producing between 5 and 100 barrels of oil per ton are considered, the oil reserve estimate is increased to about 5.5.times.10.sup.12 barrels. By way of comparison, the estimate of the world's 1975 crude oil reserve was 7.times.10.sup.11 barrels.
Oil shale underlies an estimated 20 percent of the land mass of the United States. So far as is presently known, the world's largest single oil shale reserve is in the Eocene Green River Formation which spans the states of Colorado, Utah and Wyoming. The Green River Formation, which covers about 16,500 square miles, is estimated to have an oil potential in excess of about 2.times.10.sup.12 barrels. Of this amount, about 6.times.10.sup.11 barrels are estimated to reside in deposits which are relatively producible and which yield 25 or more barrels of oil per ton of shale, an amount representing about 20 times the present estimate of this country's crude oil reserves.
It is also very important that of all the alternative oils, shale oil most closely resembles crude oil; for example, shale oil contains fewer impurities and more hydrogen than synthetic oil made from coal. Also, using present technologies, a suitable refinery feedstock can be produced from shale oil more economically than from other alternative oils.
In spite of the vast potential of the world's oil shales, the commercial production of shale oil has generally, due to high production costs, been uneconomical as compared to the cost of crude oil. Consequently, it is usually only in special circumstances, as when other fuels have been in short supply or when normal channels of crude oil transportation have been disrupted, that the extraction of oil from shale is carried out commercially. Although fairly substantial amounts of oil were produced from shale in Europe in the late 1800's and the early 1900's, the availability of low cost crude oil from the Near East and improved means of oil transportation halted most of such shale development until World War II, when the demand for oil sharply increased and the production and distribution of petroleum was disrupted. Thus, during World War II, significant amounts of shale oil were produced by Germany, France, the Soviet Union and many other countries. However, after the War, cheap crude oil again became plentiful and interest in the production of oil shale correspondingly declined once more. More recently, in the mid-1970's when Near East crises and the formation of strong oil cartels drove the cost of crude oil up sharply and oil embargos were imposed, interest in oil shale was, however, again reawakened.
It is obvious that the balance between energy supply and energy demand has in the past controlled the position of oil shale in the energy market. As long as the cost of shale oil exceeds the cost of crude oil by substantial amounts, as has usually been the case, the development of oil shale will be inhibited, unless factors other than cost, such as national interest in self-sufficiency in oil, becomes more important than the cost differential between shale and oil and crude oil.
However, in the absence of critical crude oil shortages or overriding national interests, reducing the costs associated with the production of shale oil is paramount to the continued development of oil shale reserves.
For many reasons, the production of oil from shale has remained costly. One reason is that large amounts of shale are required to produce even relatively small amounts of shale oil. For example, the a production of 10,000 barrels of oil a day from Green River oil shale may require the processing of about 12,500 tons of high grade shale a day, all of which must be mined in an environmentally acceptable manner. After being mined, the shale must typically be crushed into pieces no larger than about two inches; however; because of the high organic content of the shale crushing is difficult. Thereafter, the crushed shale must be retorted at a high temperature to decompose kerogen in the shale into raw shale oil. Typically, retort temperatures in the range of 700.degree. F. to 100.degree. F. are used in order to produce high grade oil which is almost completely aromatic with little olefin or saturate content. To be practical, such retorting must generally be done in a continuous flow manner, upflow retorts, in which the crushed shale is introduced into the bottom of the retort and retorted shale is discharged from the top, being a known apparatus for this purpose.
Significant costs are also associated with the disposal of the retorted shale, which, by weight, usually amounts to about 80 percent of the mined shale. It is to problems associated with the disposal of retorted shale that this present invention is principally directed.
A substantial difficulty with the disposal of retorted shale is that such shale still contains some residual amounts of carbon. If, therefore, the retorted shale is discharged directly into an atmospheric environment at retort temperatures, the retorted shale may spontaneously combust (assuming the retort temperature is in excess of about 500.degree. F.). For environmental and practical reasons such shale combustion in the atmosphere is undesirable and may, in fact, be unlawful in many localities. Consequently, provision is generally made for cooling the retorted shale before it is discharged for land fill, other use or disposal. For safe discharge into an atmospheric environment, the retorted shale must generally be cooled to under about 400.degree. F., as for example, in one or more vertically oriented gravity-flow cooling vessels. Retorted shale is discharged from the retort directly into the top of the cooling vessels, through which the retorted shale flows downwardly under gravity, usually in a packed bed flow. The shale is discharged from the bottom of the vessels cooled to a non-spontaneously combusting temperature. Also, water is typically sprayed onto the retorted shale as it descends through the vessels to effect the cooling process.
Retorted shale cooling vessels known to the present inventors, however, are generally deficient in that little or no shale mixing is provided and the cooling water cannot penetrate into the center of the shale flow. As a result, pockets of uncooled shale tend to become entrained in the flow of shale through the vessels. Similarly, after water injection to cool the shale, pockets of water or overwetted shale particles tend to become entrained in the shale flow. When, as frequently occurs, pockets of uncooled shale encounter pockets of water or over-wetted shale, the water violently flashes into steam, thereby causing a localized explosion in the vessels. These explosions disrupt the flow of retorted shale through the vessels and may, by the back pressure caused by the explosions, disrupt operation of the retorts which supply shale to the vessels. Even if the pockets of uncooled shale entrained in the shale flow do not encounter pockets of water or over-wetted shale, when discharged from the vessels, these uncooled shale pockets may spontaneously combust in the air. Furthermore, if the shale cooling water is not well mixed into the shale flow, the resulting localized regions of over-wetted shale may cause bridging in the discharge conduits, shale flow through the vessels may be restricted and cooling may be further impeded. Still further, the use of large amounts of water to cool the retorted shale increases the cost of shale disposal and may deplete local sources of water at an excessive rate--a problem of particular severity in the many arid regions of the western United States in which oil shale is abundant.
Problems associated with proper cooling of the retorted oil shale are caused, at least in part, by the great variation in particle size present in the retorted shale. These particle sizes may range from the maximum crush size of about 2 inches to very fine powder. Not only is the homogeneous mixing of materials having such a wide particle size range by static, gravity flow very difficult, but the very fine, powdery particles tend to inhibit the wetting of larger particles by the cooled water sprays.
It can thus be seen that improvements to retorted oil shale cooling vessels are needed to help reduce the costs of producing oil from oil shade.
An object of the present invention is, therefore, to provide a gravity flow, mixing tube apparatus for mixing particulate matter, the apparatus having a plurality of three dimensional internal, static mixing members configured for providing efficient particulate matter mixing in a comparatively short tube length.
Another object of the present invention is to provide a gravity flow, mixing tube apparatus for mixing and cooling hot particulate matter, such as hot retorted oil shale, the apparatus having a plurality of mixing tubes each having a plurality of three-dimensional, static mixing members and having means for introducing cooling water into the tubes and means for enabling disengagement of steam produced by the water contacting the hot particulate matter from the matter and the tubes.
Still another object of the present invention is to provide a three-dimensional particulate matter mixing member adapted for use in gravity flow, static mixing tube.
Other objects, features and advantages of the present invention will be readily apparent from the following detailed description thereof when taken in conjunction with the accompanying drawings.